1. Field of the Invention
The present invention relates generally to surgical instruments, and is particularly concerned with an ophthalmic surgical instrument having an asymmetrically tapered or beveled tip and a method for making said instrument.
2. Description of the Prior Art
Endophthalmic surgery, or surgery on an intact and normally pressurized eye, represents an important and relatively recent development in the field of ophthalmology. In this technique, the existing optical pathways of the pressurized ocular globe are utilized for visualization during delicate intraocular manipulations. Maintenance of positive intraocular pressure tends to preserve and stabilize the spatial relationships among the various intraocular tissues.
The archetypal endophthalmic procedure is the cataract aspiration technique of extracapsular cataract surgery described by Scheie, Am. J. Ophthal. 50:1048 (1960), wherein an instrument is passed through a small incision at the margin of the cornea into the anterior aqueous chamber of the eye to incise the anterior capsular membrane of the lens. The aqueous, which leaks out during this manipulation, is replaced by a gravity-fed infusion of physiologic saline through a cannula inserted into the anterior chamber through a second small incision. A blunt needle is then inserted through the first incision into the lens, whereupon gentle suction aspirates the soft lens substance leaving the posterior capsular membrane in place. Whatever volume is removed or leaks from the two small incisions is replaced by the continuous gravity feed of saline. Absent any seepage or applied suction, the pressure in the eye stabilizes at a point determined by the physical elevation of the saline column above the level of the eye. At the end of the procedure, all tubes are withdrawn and the incisions are sutured.
Conceptually, all modern endophthalmic procedures are variations and refinements of the foregoing technique. In pars plana vitrectomy, for example, an incision is made through the scleral coat of the eye in the pars plana region between the iris and the anteriormost retina. A tract is cut with a long, sharp knife into the vitreous, and the tip of the vitrectomy instrument is placed in the eye. Infusion is provided through a separate incision or through cannula concentric with the vitrectomy instrument itself. Gentle aspiration provided through the tip of the vitrectomy instrument engages vitreous or other intraocular tissues, which are then sheared or sliced in small bits and removed from the eye through an aspiration tube. Illumination is provided by optical fibers which may be concentric with the vitrectomy instrument or inserted separately. The procedure is visualized through the dilated pupil with a high magnification operating microscope. Actuation of the various functions at the instrument tip is usually by remote foot control.
The earliest powered vitrectomy instruments utilized rotary cutting elements. See, e.g., Machemer et al., Trans. Am. Acad. Ophthal. Otolaryng. 75:813 (1971). Rotary instruments, however, tended to cause undesirable tangential pulling or shearing of the tissue being severed, particularly when the rotary cutting element became dull. This sometimes resulted in partial wrapping or spooling of the uncut tissue around the rotating cutting element. Efforts to avoid these effects led to the development of linearly reciprocating cutting instruments, an early example of which is described by Peyman et al., Arch. Ophthal. 86:548 (1971). The instrument portion of the Peyman et al. handpiece consists of two concentric tubes with a hole near the distal end of the outer tube. Cutting is performed by the chopping action of the sharpened end of the inner tube against the plane interior end of the outer tube. Suction applied to the inner tube urges the tissue to be severed into the hole in the outer tube and then removes the severed bits of tissue from the eye. Infusion is provided through a small tube running parallel to the outer concentric tube. The necessary powered reciprocation of the inner tube relative to the fixed outer tube is provided by a small electrical solenoid, the oscillation rate of which can be varied. A description of this handpiece can also be found in U.S. Pat. No. 3,776,238, to Peyman et al.
More recently, pneumatic devices have replaced electrical solenoids as the source of linear reciprocating motion for the inner tube of the cutting instrument. This development is reflected, for example, in Peyman et al., Am. J. Ophthal. 75:774 (1973), and in U.S. Pat. Nos. 3,815,604, 3,884,237 and 3,884,238, all to O'Malley et al. Pneumatic devices are readily adaptable to linear reciprocating motion, do not inherently generate heat, and can be constructed from lightweight materials. Additional advantages of pneumatic power sources are a more evenly modulated power pulse and elimination of the potential electrical hazard presented by electrical solenoid devices.
Another relatively recent development, also reflected in the previously-cited U.S. patents to O'Malley et al., is the substitution of a shear-type cutting instrument for the earlier chopping cutter disclosed by Peyman et al. In the shear-type cutter, the cutting action takes place between the sharpened distal edge of the inner tube and the sharpened edge of a hole or port formed near the distal end of the outer tube. The shear-type cutter produces less wear on the sharpened edge of the inner tube than is the case with the chopping cutter, and is the type of cutter which is used most frequently at the present time.
Many of the problems experienced with the early vitrectomy instruments were related to the use of bulky combination probes, which often required concentric infusion, cutting and illumination instruments to be inserted into the vitreous body through a single scleral incision. Typically, the scleral incision is linear and has a length approximately equal to one-half the circumference of the instrument probe, in order to achieve a fluid-tight fit. The instrument tip is introduced with a twisting motion; often this is preceded by the insertion of a beveled hypodermic needle of the same diameter as the probe. The concentric fiber-optic illumination or infusion sleeves with sharp "shoulders" would occasionally catch on the elastic choroidal layer or adherent vitreous base, causing choroidal separation or retinal tears. These problems have been minimized by miniaturization, streamlining, and specialization of the intraocular probes. For example, it is common at the present time to insert the infusion cannula and illumination fiber into the vitreous body through one or more separate scleral incisions, rather than by attaching these devices to the vitrectomy instrument probe.
Improved instrumentation has led to more intricate and complex manipulation near, on, and beneath the retinal surface. One of the basic rules of endophthalmic surgery within the vitreous is that the distal cutting or aspiration port must always be visible, so that the surgeon can insure that only material which is intended to be cut enters the port. The ability to cut close to the retinal surface with constant visualization of the cutting port is a highly desirable feature of vitrectomy tip design. This advantage is realized when the cutting port is located as close as possible to the instrument tip, and when the configuration of the instrument tip is such that the port can be positioned as close as possible to the retinal surface while still remaining within the surgeon's line of sight.
For the most part, the vitrectomy instruments which are available at the present time have two different types of tip configurations. The first type, exemplified by U.S. Pat. No. 3,994,297, to Kopf, and U.S. Pat. No. 4,011,869, to Seiler, Jr., is characterized by a symmetrically rounded tip shape which is essentially hemispherical. Similar tip shapes can be found in the above-mentioned U.S. Pat. Nos. 3,815,604, 3,884,237 and 3,884,238, all to O'Malley et al. Although this type of tip shape facilitates penetration of the eye through the scleral incision, it suffers from the disadvantage that the cutting port in the outer tube must be located relatively far back from the instrument tip. The reason for this is that the vertically cut distal end of the inner cutting tube cannot be accommodated in the area of reduced cross-section defined by the rounded distal end of the outer tube. As a result, the forward stroke of the inner tube must terminate before the point where the outer tube begins to taper. The second type of tip configuration, exemplified by U.S. Pat. No. 4,099,529, to Peyman, and U.S. Pat. No. 4,111,207, to Seiler, Jr., is characterized by a flat tip shape which is formed essentially by closing off the distal end of the outer tube with a plane vertical wall. This arrangement allows the cutting port to be located relatively close to the instrument tip, but only at the expense of a blunt instrument tip which does not readily penetrate the eye.
In U.S. Pat. No. 4,210,146, to Banko, a surgical instrument is disclosed in which the inner cutting member consists of a flexible blade carried by an axially reciprocating drive shaft. As the shaft moves forward, the flexure of the blade allows it to move into the symmetrically rounded and tapered instrument tip, where the cutting port is located. This arrangement allows the cutting port to be located somewhat closer to the instrument tip than would be possible with a rigid inner cutting tube, although this advantage is offset by the need to substitute a precisely machined flexible blade and drive shaft arrangement for the simple rigid tube that is conventionally used as the inner cutting member.
Another basic technique in endophthalmic surgery is the use of spatulated instruments or picks to elevate or dissect periretinal and epiretinal membranes prior to excising them. The round or blunt tips of most currently available vitrectomy instruments are not particularly useful for these maneuvers. For this reason, it is common to employ a separate instrument, such as a hooked needle, to elevate the membrane to a point where it can be engaged and excised with the vitrectomy instrument tip. A vitrectomy instrument with a beveled tip would be more desirable in this situation, since the sharp distal edge at the end of the beveled tip might allow the instrument itself to be used as a spatula for elevating or dissecting intraocular material prior to cutting. The only known example of a vitrectomy instrument with a beveled tip is that shown by Peyman et al., Am. J. Ophthal. 75:774 (1974), at FIG. 5D, although the taper angle of this particular instrument tip is somewhat steeper than would be desired for convenient manipulation of the intraocular material to be excised. Also, in view of the rather substantial spacing between the cutting port and the distal end of the outer tube, the Peyman et al. instrument tip would not provide the advantage referred to earlier, that is, the ability to sever intraocular material positioned on or very close to the surface of the retina. Peyman et al. have reported other difficulties with this tip design, in particular, a limited blade life and a gradual enlargement of the spacing between the inner cutting tip and the outer tube to a point where cutting of the fiber vitreous bands became difficult or impossible.